Trace analyte detection is the detection of small quantities of analytes, often at nanogram to picogram levels. Trace analyte detection can be particularly useful for security applications such as screening individuals or items for components in explosive materials, narcotics and biological warfare agents, where small amounts of these components are deposited on the individual or on the outside of a package or bag. Inherent in the detection of a small quantity of analyte is the possibility of unacceptably high false alarm rates.
Ion mobility spectrometry (IMS) is a well-established technique for the detection of narcotics and explosives. However, due to the sensitivity of the IMS technique, interferences can occur and cause false alarms. If the rate of false alarm is excessive, the practical value of the instrument is diminished due to lack of confidence in the instrument's performance.
False alarms can be caused by detection of trace quantities of an analyte from another source. False alarms can also be caused by chemical signature interferences between substances, such as cosmetics, toiletries, and the like, and an analyte of interest, making it difficult to differentiate an analyte of interest from an interferant.
One way to reduce false alarm rates is by modification of IMS hardware and software. Another means for increasing detection specificity and, thus, reducing false alarm rates is by adjusting the ionization chemistry employed in the IMS by using a different ionization reagent. During the collection of IMS data, a sample is collected, the sample, which may contain an analyte of interest, is introduced into the IMS instrument and then vaporized. The vaporized analytes are directed to an ionization chamber. In the ionization chamber, the analytes are ionized via proton or electron-transfer from ionized reactants. The character of the ionized reactants can affect the type and distribution of analyte ions, altering the IMS signature of an analyte. Reactants can affect the number and size of peaks an analyte produces in an IMS signature and can suppress peaks entirely.
An optimal ionization reagent is capable of forming readily identifiable analyte ions, while suppressing ion formation for molecules which are not of interest. Commonly used IMS reagents include, for example, nicotinamide, ammonia, hexachloroethane, chlorine, 4-Nitrobenzonitrile and water, acetone, and bominated or chlorinated hydrocarbons.
Using common IMS ionization reagents, certain analytes of interest, such as peroxide-based explosives, can be difficult to detect or result in false positive results because this commonly used ionization chemistry does not allow for accurate detection of these explosives.